The chest x-ray is the most commonly performed diagnostic x-ray examination. The heart, lungs, airway, blood vessels and the bones of the spine and chest are imaged in a painless medical test that helps in the diagnosis of medical conditions.
The chest x-ray is typically the first imaging test used to help diagnose causes of symptoms such as shortness of breath, fever, a bad or persistent cough, chest pain or injury. Its application helps in diagnosing and monitoring treatment for medical conditions such as pneumonia, lung cancer, emphysema and heart failure and other heart problems. It may be used to find fractures in ribs as well.
Pneumonia shows up on radiographs as patches and irregular whiter areas, indicative of higher x-ray absorption from fluid in the lungs. If the bronchi, which are usually not visible, can be seen, a diagnosis of bronchial pneumonia may be made. Shifts or shadows in the region corresponding to the hila (the central mediastinal surfaces of the lungs where the lung roots enter) may indicate emphysema or a pulmonary abscess. Apparent widening of the spaces between ribs suggests emphysema. Other pulmonary diseases may also be detected or suspected through chest x-ray examination.
Lung cancer usually shows up as some sort of abnormality on the chest radiograph. Hilar masses (enlargements at that part of the lungs where vessels and nerves enter) are one of the more common symptoms as are abnormal masses and fluid buildup on the outside surface of the lungs or surrounding areas. Interstitial lung disease, which is a large category of disorders, many of which are related to exposure of substances (such as asbestos fibers), may be detected on a chest x-ray as fiber like deposits, often in the lower portions of the lungs.
The lungs are situated within the rib cage. Because bones absorb x-rays to a larger extent than soft tissues, these appear as white stripes across the x-ray radiograph. The ribs curl around the body and consequently the rib shadow on the x-ray radiograph appear to cross each other. The cross-over points and the regions between the cross-over points appear as a pattern of almost parallelogram-like shapes. This pattern may disguise the shadows cast by nodules which may be aligned with the crossover points, with the spaces therebetween, with a single rib between the cross-over points, and frequently overlap two or three of these regions.
In addition to the rib cage, the blood vessels within the lung have higher density than the air filled spongy lung tissue and absorb more x-rays, thereby causing them to cast a shadow, i.e. appear lighter on the radiograph. The lungs themselves have edges and absorb more x-rays than the space in the thoracic cavity therearound.
Absolute absorption varies with the period of exposure, the strength of the x-ray source and the size of the patient. Details are resolved by contrast between regions, and thus CAD programs seek out and identify edges.
Although supposed to keep still and hold their breadth, keeping lungs inflated, patients may move or inhale or exhale during the exposure to x-rays, blurring the resultant radiography image somewhat. Since x-ray radiation may itself cause malignancies, radiologists are wary of repeatedly exposing patients to x-rays, particularly with pregnant women, the young, the elderly and/or the ill. With some systems, such as those using traditional film, it may be impossible to repeat the test once a problem is determined, as the patient is no longer available.
Lung cancer may manifest itself as one or more malignant nodule which is typically near-spherical and shows up as a near-circular whiter region on the lung tissue. The size of the nodule may be smaller, similar or larger than the rib cross-over, and thus isolation and identification of nodules is not easy.
It is, of course, imperative to detect all resolvable nodules. To avoid unnecessary anxiety and to reduce costs of unnecessary biopsies; it is desirable to minimize false positives as well.
The skilled radiographer may manually pick out nodules, but to speed up the diagnostic procedure, Computer Aided Diagnostic (CAD) techniques are increasingly used.
One approach used is segmentation, where the borders of features such as the lung lobe itself and ribs are identified. This is not always possible and indeed, is not always necessary.
What is required is to identify candidate regions of interest and to then decide if they are nodules or false positives. Once identified as a nodule, it is then necessary to determine if it is benign or malignant. This may require a biopsy, but can sometimes be determined by monitoring the nodule over time.
‘False negatives’ where actual nodules or tumors are not identified as such or discarded as ‘noise’ or some artifact of the system are unacceptable. The occurrence of ‘false positives’ where some other feature such as a rib crossing shadow is mistakenly identified as being attributed to a tumor, should be minimized. In practice the elimination of false positives whilst avoiding false negatives is difficult since tumors may have similar dimensions to the width of ribs or the diameter of blood vessels within the lung, and x-ray images typically have poor contrast. When scanning the lung region of posterior anterior x-ray images to look for regions of varying contrast or edges, a large number of candidate regions turn out to be rib crossings and the like.
There is a need to provide alternative approaches and methods to computer aided diagnostics for examining chest x-rays and the present invention addresses this need.